Hi Rick,
This blog is really so helpful and appreciated. Your code for plotting linear gression with RCS works very well when there are no covariates, but how can I visualize the multivaraible-adjusted spline. Here is my code. I found this code only plots unadjusted predicted value.

proc glmselect data=diet outdesign(addinputvars fullmodel)=SplineBasis;
effect splT_fish= spline(raw_T_fish/ basis=tpf(noint) NATURALCUBIC details knotmethod=percentilelist(5 27.5 50 72.5 95));
model n3pufas=splT_fish AGE SEX BMI raw_cereals raw_pulses raw_meat raw_egg raw_milk
raw_potato raw_fruits raw_veg nuts vegoilfat seaweeds energy /selection=none;

proc reg data=SplineBasis plots=none;
model n3pufas=splT_fish_: ;
output out=regout predicted=fit LCLM=Lower UCLM=Upper;

proc sgplot data=regOut noautolegend Noborder nowall;
scatter x=raw_T_fish y=Fit;
series x=raw_T_fish y=Fit ;
Series y=Lower x=raw_T_fish ;
Series y=Upper x=raw_T_fish;